1. Field of the Invention
This invention relates to improvements in metal detecting devices. More specifically, it relates to improvements in beat frequency oscillator type metal detectors that provide limited discriminatory capability, enhanced sensitivity, ease of operation and useful features at a low cost.
While the present invention is described herein with reference to particular embodiments, it should be understood that the invention is not limited thereto. The metal detector of the present invention may be adapted and employed for a variety of requirements as those skilled in the art will recognize in light of the present disclosure.
2. Description of the Prior Art
Metal detecting devices of various designs are widely used in the industry. Exemplary of such devices are those shown in the following U.S. Patents: Pat. No. 2,393,717, issued to Speaker on Jan. 29, 1946; Pat. No. 3,325,739, issued to Stephenson on June 13, 1967; Pat. No. 3,467,855, issued to Rance on Sept. 16, 1969; Pat. No. 3,986,104, issued to Randolph on Oct. 12, 1976; Pat. No. 3,355,658, issued to Gardiner on Nov. 28, 1967; Pat. No. 3,769,575, issued to Rist et al. on Oct. 30, 1973; Pat. No. 3,626,279, issued to Walden on Dec. 7, 1971; Pat. No. 2,230,502, issued to Pearson on Feb. 4, 1941; and Pat. No. 3,909,704, issued to Schonstedt on Sept. 30, 1975. Further, How To Build Proximity Detectors And Metal Locators, By J. P. Shields, p. 121 (2nd Ed., 1972) is also instructive.
Previously known metal detectors have most often been of one of three types: (1) discriminator, (2) ground neutralization, and (3) beat frequency oscillator (BFO). Discriminator type metal detectors, as typified by the above-mentioned patent to Randolph, No. 3,986,104, are effective to select valuable metals from other metals and utilize the principle that valuable metals such as gold, silver and copper have relatively high conductivity and, as a result, low resistance to eddy currents induced by a search coil. Typically, the eddy currents set up a counter-acting magnetic field which reduces the inductance of the coil. Lower conductivity metals such as steel, iron, aluminum, lead, etc. also produce eddy currents, but reflect higher resistive losses back to the search coil. Discriminators utilize differences in reflected resistive losses to provide discriminative information to the operator.
Ground neutralization type metal detectors are similar to discriminator type detectors but have the capability to neutralize the effect of permeable soil high in mineral, i.e. iron, content. Such detectors generally use some method of combining amplitude and phase shift information between a transmit and receive search coil to yield a signal that does not change with varying proximity to a permeable background. Various circuit techniques have been used to achieve this.
BFO type metal detectors, as typified by the above-mentioned patents to Speaker, No. 2,393,719; Stephenson, No. 3,325,739; Gardiner, No. 3,355,658; Rist et al., No. 3,769,575; Walden, No. 3,626,279; Pearson, No. 2,230,502 and Schonstedt, No. 3,909,704 are employed to find metal where there appears to be none. BFO detectors take advantage of the fact that most conductive or metallic objects, being susceptible to eddy currents, will cause a decrease in the inductance of the search coil and a corresponding increase in the oscillating frequency of a search oscillator, whereas permeable minerals, being unsusceptible to eddy currents, will cause an increase in the inductance of the search coil and a corresponding decrease in the oscillating frequency of the search oscillator. BFO type metal detectors typically examine the difference between the frequency of the search oscillator and that of a fixed frequency reference oscillator and give the user an aural or visual indication of the difference frequency. The user then interprets any changes in the difference frequency as an indication of the presence of a conductive or permeable substance depending on the direction of the frequency shift as the search coil approaches it.
BFO type metal detectors have, in the past, had an undesirable ambiguity in the indication provided thereby under at least some circumstances. In particular, the difference frequency produced thereby increases as the frequency of the search oscillator increases or decreases from the point at which the search oscillator and reference oscillator frequencies are equal. If the search and reference oscillator nominal operating frequencies are sufficiently close together that the actual operating frequencies become equal during operation of the detector, a shift in the frequency of the search oscillator through the zero beat frequency yields a misleading output signal. The output is confusing in that the operator, often unaware of the direction of the frequency shift, will be lead to believe that, for example, a conductive object is being detected when in actuality the search oscillator has rapidly moved to the other side of the zero beat frequency such that the device is sensing a permeable mineral. Thus, a problem has existed as to how to determine whether the output signal corresponds to a beat signal that is at a greater or lesser frequency than that of the zero beat frequency.